Solve for $y$ : $15 = 9 + y$
Explanation: Subtract $9$ from both sides: $ \begin{eqnarray} \\ \hphantom{15 {- 9}} &\hphantom{=}& \hphantom{y} \\ 15 &=& 9 + y \\ \\ {-9} && {-9} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 15 {- 9} &=& y \\ \end{eqnarray} $ Simplify. $ y = 6$